N×M digitally programmable optical routing switch

ABSTRACT

An optical routing switch provides polarization-independent and low-crosstalk switching between any of a plurality of input ports and any of a plurality of output ports over a wide operating range of temperatures and wavelengths. Optical signals appearing at each input port are spatially decomposed into two orthogonally-polarized beams by a first polarization-dependent routing element (e.g., a birefringent element or polarized beamsplitter). Beyond this point, a network of optical switches are placed along the optical paths of the pair of light beams. Each optical switch includes: (1) a polarization rotator that switchably controls the polarization of the input light beams so that both of the emergent beams are either vertically or horizontally polarized, according to the control state of the device; and (2) a polarization-dependent routing element that spatially routes the light beam pair to provide physical displacement based on their state of polarization. The final stage for each output port in the network consists of an array of polarization rotators that changes the polarization of at least one of the light beams, so that the two beams become orthogonally polarized. Finally, a polarization-dependent routing element (e.g., a birefringent element) intercepts the two orthogonally-polarized beams and recombines them to exit at the selected output port.

RELATED APPLICATION

The present application is a continuation of Applicants' copending U.S.patent application Ser. No. 09/063,611 entitled “N×M DigitallyProgrammable Optical Routing Switch,” filed on Apr. 21, 1998, ofApplicants' U.S. Pat. No. 6,049,404, entitled “N×M DigitallyProgrammable Optical Routing Switch” which is a continuation-in-part ofSer. No. 08/979,525 the Applicants' U.S. Pat. No. 5,946,116, entitled“1×N Digitally Programmable Optical Routing Switch” filed Nov. 26, 1997,which is based on a U.S. Provisional Patent Application No. 60/042,575,entitled “1×2^(N) Digitally Programmable Optical Routing Switch” filedApr. 2, 1997.

GOVERNMENT INTERESTS

The invention was made with Government support under Contract BMDOI:DASG60-97-M0081 awarded by U.S. Army Space & Strategic DefenseCommand, CONT AND ACO MGMT OFC, CSSD-CM-CT, P.O. Box 1500, Huntsville,Ala. 35807. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to switching of optical signals; and inparticular, to the spatial routing of optical signals transmitted inoptical communication networks and optical signal processing.

2. Background of the Invention

Optical fibers are used as the physical media for transmitting opticalsignals in a variety of commercial and military applications. As thedata rates of information continue to grow, it becomes increasinglydifficult for conventional electronic switching systems to handle higherbandwidths. In addition, the required conversion between optical andelectrical signals restricts the data format and increases costs.All-optical routing/switching technologies, characterized by high “datatransparency,” can switch or transfer optical signals from onetransmission channel to another while the signals remain in opticalform.

Several multiplexing schemes have been developed in fiber opticinterconnection networks, including time-division multiplexing (TDM),wavelength-division multiplexing (WDM) and space-division multiplexing(SDM). Space-division switching is considered to be one of the mostimportant fiber optic routing schemes. Major applications ofspace-division photonic switches are in fiber optic communicationnetworks, optical gyroscopes, optical signal processing, andmicro/millimeter wave signal distribution for phased-array radarsystems.

A wide variety of electromagnetic field-controlled optical switches arecommercially available. They are based on mechanical, electro-optic,thermo-optic, acousto-optic, magneto-optic, and semiconductortechnologies. Each switching technology has its own advantages, but alsohas drawbacks as well. For example, mechanical switches are the mostwidely used routing components and provide very low insertion loss andcrosstalk characteristics, but their switching time is limited to themillisecond range. They also have a limited lifetime becausemotor-driven parts are used. LiNbO₃ integrated optic switches, on theother hand, offer nanosecond switching times. However, LiNbO₃ switchessuffer from the disadvantages of relative large insertion loss (5 dB),high crosstalk (20 dB) and polarization dependency.

Accordingly, efforts continue to develop field-controlled opticalswitches with lower channel crosstalk, reduced polarization dependentloss, and at least moderate reconfiguration speed. It is recognized thatthese efforts, when successful, can provide an essential component tofiber communication systems.

3. Solution to the Problem

The present invention employs an optical network of polarization rotatorarrays and polarization-dependent routing elements (e.g., birefringentelements or polarized beamsplitters) to achieve an optical routingstructure that provides polarization-independent and low-crosstalkswitching over a wide operating range of temperatures and wavelengths.This optical switch retains the switched signals in optical format andpreserves their optical properties.

SUMMARY OF THE INVENTION

This invention describes an optical routing switch for selectivelyrouting an optical signal from any of a plurality of input ports to anyof a plurality of output ports. The optical signal at each input port isspatially decomposed into two orthogonally-polarized beams by a firstpolarization-dependent routing element (e.g., a birefringent element orpolarized beamsplitter). Beyond this point, a network of opticalswitches are placed along the optical paths of the pair of light beams.Each optical switch includes: (1) a polarization rotator that switchablycontrols the polarization of the input light beams so that both of theemergent beams are either vertically or horizontally polarized,according to the control state of the device; and (2) apolarization-dependent routing element that spatially routes the lightbeam pair to provide physical displacement based on their state ofpolarization. The final stage for each output port in the networkconsists of an array of polarization rotators that changes thepolarization of at least one of the light beams, so that the two beamsbecome orthogonally polarized. Finally, a polarization-dependent routingelement (e.g., a birefringent element) intercepts the twoorthogonally-polarized beams and recombines them to exit at the selectedoutput port.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more readily understood in conjunction withthe accompanying drawings, in which:

FIG. 1 is a block diagram of a 1×2^(N) optical switch in accordance withpresent invention.

FIGS. 2a and 2 b are block diagrams of two preferred architectures foroptical routing switches in accordance with the present invention.

FIGS. 3a through 3 d are block diagrams of a 1×4 routing switch designedin a 2-dimensional structure based on FIG. 2b. FIGS. 3a through 3 dillustrate the light paths for the input optical energy coupled to eachof the selected output ports in the four control states of the switch.

FIGS. 4a through 4 d are block diagrams of a 1×4 routing switch designedin a 2-dimensional structure based on FIG. 2a. FIGS. 4a through 4 dillustrate the light paths for the input optical energy coupled to eachof the selected output ports in the four control states of the switch.

FIGS. 5a through 5 d are diagrams of a 1×4 routing switch using a3-dimensional structure based on FIG. 2a. FIGS. 5a through 5 dillustrate the light paths for the input optical energy coupled to eachof the selected output ports in the four control states of the switch.

FIGS. 6a through 6 d are diagrams of a 1×4 routing switch using a3-dimensional structure based on FIG. 2b. FIGS. 6a through 6 dillustrate the light paths for the input optical energy coupled to eachof the selected output ports in the four control states of the switch.

FIGS. 7a through 7 d are diagrams of a 1×4 routing switch using a3-dimensional structure in which all the birefringent elements have thesame thickness.

FIG. 8 is a diagram of an alternative embodiment of a 1×5 routing switchusing polarized beamsplitters in place of birefringent elements.

FIG. 9 is a diagram of another embodiment of a 1×8 routing switch usinga tree structure of angled polarized beamsplitters.

FIG. 10 is a diagram of another alternative embodiment of a 1×8 routingswitch using polarized beamsplitters.

FIG. 11 is a diagram of another alternative embodiment of a 1×4 routingswitch using angled polarized beamsplitters.

FIG. 12 is a diagram of another alternative embodiment of a series offour 1×4 routing switches stacked atop one another.

FIG. 13 is a cross-sectional diagram of an alternative embodiment of apolarization-dependent routing element combining a PBS with a reflectiveprism.

FIG. 14 is a cross-sectional diagram of a 2×2 routing switch using apolarized beamsplitter.

FIG. 15 is a cross-sectional diagram of an alternative embodiment of a2×2 routing switch using a network of polarized beamsplitters.

FIG. 16 is a cross-sectional diagram of a 2×8 routing switch usingpolarized beamsplitters.

FIG. 17 is a cross-sectional diagram of a 4×4 routing switch.

FIG. 18 is a cross-sectional diagram of an alternative embodiment of a4×4 routing switch.

FIG. 19 is a cross-sectional diagram of a 6×6 routing switch.

FIGS. 20a and 20 b are diagrams of the two control states of analternative embodiment of a 2×2 routing switch.

FIG. 21 is a diagram of an alternative embodiment of a 4×4 routingswitch.

FIGS. 22a through 22 d are diagrams of another alternative embodiment ofa 2×2 routing switch.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the general concept of a 1×2^(N) optical routingswitch. An optical signal is input through an input port 500 and passesthrough a birefringent element (or polarization beam separator) 30. Thisbirefringent element 30 decomposes the light beam into two componentshaving orthogonal polarizations (e.g., horizontal and vertical). The twobeams are also spatially separated by the first birefringent element 30due to the birefringent walk-off effect. In FIG. 1, thin lines representone state of polarization and thick lines represent the second,orthogonal polarization. The beams pass through a first array ofpolarization rotators 100, which consist of two sub-elements (or pixels)that intercept the two beams. The polarization rotator array 100converts the polarization of one of the light beams, so that both beamshave the same polarization when they exit the first polarization rotatorarray 100.

Both light beams then pass through a second birefringent element 301that directs the light beams based on their polarization, due to thebirefringent walk-off effect. At the output of the second birefringentelement 301, there are two possible spatial positions for each of thelight beams (drawn as the solid thin lines and the dashed thin linesafter the second birefringent element 301) based on their polarizationentering the second birefringent element 301. The two beams then passthrough another array of polarization rotators 900 that are divided intotwo sub-elements as shown in FIG. 1. Based on the control states of thesub-elements in the polarization rotators 100 and 900, the pair of lightbeams exiting the second array of polarization rotators 900 can haveeither of two different polarizations (e.g., horizontal or vertical) andeither of two possible spatial positions, thus resulting in fourpossible combinations of polarization and spatial position.

These four combinations are spatially separated using a thirdbirefringent element 302. More specifically, the four possiblecombinations of polarization and position at the input plane of thethird birefringent element 302 are separated into four possible spatialpositions at its output plane due to the birefringent walk-off effect.

This combination of a birefringent element with an array of polarizationrotators can be repeated in an arbitrary number of stages stacked inseries along an optical axis. With N stages of the birefringent elements301, 302, . . . 30n−1, 30n and polarization rotators 900, 901, . . .90n−1, 90n inter-digitally placed together (i.e., 301/900, 302/901, . .. 30n−1/90n−1, and 30n/90n as shown in FIG. 1), there are total of 2^(N)possible output positions for the original pair of beams entering thefirst stage.

The final polarization rotator array 90n converts the beam pair back toorthogonal polarizations. This is indicated by the thin and thick linesafter the final polarization rotator array 90n, where thin linesrepresent one polarization and thick lines represent the orthogonalpolarization. The orthogonally-polarized beam pair are combined by afinal birefringent element 60 and exit at one of the 2^(N) output ports.

To help make the design fault tolerant, the thicknesses of thebirefringent elements can be varied in a geometrical order, as shown inFIGS. 2a and 2 b. In FIG. 2a, the birefringent elements have thicknessesof L, L/2, . . . , L/2^(N−1), and L/2^(N). In contrast, the order isreversed in FIG. 2b and the birefringent elements have thicknesses of L,2L, . . . , 2^(N−1)L, and 2^(N)L. The principle of operation isgenerally the same as described in FIG. 1. These variations in thethicknesses of the birefringent elements help maintain beam separationfor the beam pair as they pass through each stage. A total of 2^(N)possible beam positions exist at the exit plane of final stage (i.e., atthe birefringent element 30n). Therefore, the final polarizationcontroller 90n must have 2^(N+1) pixels so that each of the 2^(N)possible beam pairs can be converted to orthogonal polarizations. Theconfigurations shown in FIG. 2a and 2 b help to provide sufficient beamseparation so that the final polarization rotator array 90n can bepixelized for each of the possible output positions. This arrangementcan then block the leakage of the light at each of the 2^(N+1) possiblebeam positions, which if passed, would cause cross-talk at undesiredoutput ports.

Alternatively, the embodiments shown in FIGS. 1, 2 a and 2 b can also beviewed as a binary tree structure composed of a series of opticalswitching stages. Each stage includes: (a) a polarization rotator array100, 900, 901, etc., selectively rotating the polarization of the inputbeam pair so that both beams have the same polarization determined bythe control state of the switch; and (b) a birefringent element 301,302, etc., selectively routing the beam pair to a selected one of thepossible output beam positions determined by their polarization.

In particular, the orthogonally-polarized pair of beams exiting thefirst birefringent element 30 are received by the first stage 100, 301.Thereafter, the Nth stage receives the beam pair in a selected one of ₂^(N−1) possible input beam positions from the preceding stage anddirects the beam pair to any of 2^(N) possible output beam positionsdetermined by the control state of the pixels in the polarizationrotator array for the state. A final polarization rotator array rotatesthe polarization of the beam pair exiting the last stage so that thebeams are orthogonally polarized and can be combined by the finalbirefringent element 60 at a selected one of the output ports

2-D DESIGN OF A 1×4 OPTICAL ROUTING SWITCH. A two-dimensional design ofan optical routing switch is illustrated in FIGS. 3a through 3 a. Asbefore, light entering through from the input port 500 is split into twoorthogonal polarizations by the first birefringent element 30. Theoptical axis of the birefringent element 30 is oriented obliquely to thelight beam propagation direction such that the optical input isdecomposed into a pair of orthogonally-polarized beams. The firstpolarization rotator array 100 is divided into two sub-elements withcomplementary states, i.e., when one is on and the other is off. Thisarrangement makes both light beams become either vertically orhorizontally polarized at the exit plane of the first polarizationrotator array 100. The circular dots and short parallel lines in FIGS.3a through 3 a represent vertical polarization and horizontalpolarization, respectively.

FIG. 3a depicts the optical routing switch configured to route the inputsignal to output port 501. In FIG. 3a, the first polarization rotatorarray 100 is set to rotate the vertically-polarization beam tohorizontal polarization, so that both light beams have horizontalpolarization when they exit the first polarization rotator array 100.These horizontally-polarized beams are redirected upward in the secondbirefringent element 40 because they are extra-ordinary waves in thisbirefringent element 40. The two beams then enter a second array ofpolarization rotators 101 having two sub-elements. In FIG. 3a, thesecond polarization rotator array 101 is set to provide no polarizationrotation and the light beams keep their horizontal polarization. Thebeams then enter a third birefringent element 50, that has a thicknesstwice that of the second birefringent element 40. Here again, the beamspropagate upward and exit at the highest level of the third birefringentelement, because they are the extraordinary wave in this birefringentelement 50. These two beams continue to have the same polarization asthey reach the third array of polarization rotators 102. This array 102has four pairs of pixels or sub-elements. As shown in FIG. 3a, one ofthe sub-elements is set to convert one of the beams to verticalpolarization so that the beam pair becomes orthogonally polarized again.These two orthogonal beams are recombined by a fourth birefringentelement 60 and exit at output port 501.

FIG. 3b shows the switch configured to couple input port 500 to outputport 503. Here, the upper sub-element of the second polarization rotatorarray 101 rotates the polarizations of both beams by 90° so that theirpolarizations become vertical. These two vertically-polarized beams areconsidered as ordinary waves in the third birefringent element 50.Therefore, no deviation occurs and the beams travel straight through thethird birefringent element 50. The two vertically-polarized beams areintercepted by the third polarization rotator array 102, which convertsone beam to horizontal polarization. The resultingorthogonally-polarized beams are recombined by the fourth birefringentelement 60 and exit at output port 503.

FIG. 3c shows the switch configured to couple the input port 500 tooutput port 502. Here, the control states of the sub-elements in thefirst polarization rotator array 100 are reversed in contrast to FIG.3a, so that both beams are vertically polarized. Thevertically-polarized beams are considered to be ordinary waves in thesecond birefringent 40, and therefore propagate straight through thisbirefringent element 40. The second polarization rotator array 101 isset to rotate the polarizations of both beams by 90°, so that theybecome horizontally polarized. These two horizontally-polarized beamsare considered as extraordinary waves in the third birefringent element50, and therefore travel upward within the birefringent element 50. Bothbeams are intercepted by the third polarization rotator array 102, whichconverts one of the beams to vertical polarization. The resultingorthogonally-polarized beams are recombined by the fourth birefringentelement 60 and exit to output port 502.

FIG. 3a shows the switch configured to couple the input port 500 tooutput port 504. Here, the second polarization rotator array 101 is setto provide no polarization rotation, so that the two light beamsmaintain their vertical polarizations. These two vertically-polarizedbeams are considered as ordinary waves in the third birefringent element50, and therefore travel straight through this birefringent element 50.The two vertically-polarized beams are intercepted by the thirdpolarization rotator array 102, which changes the polarization of one ofthe beams to horizontal. The resulting orthogonal beams are recombinedby the fourth birefringent element 60 and exit to output port 504.

FIGS. 4a through 4 d show another 2-D embodiment for a 1×4 opticalswitch. Here, the second and third birefringent elements are reversed sothat the thicker element is closer to the input port 500. When the firstpolarization rotator array 100 is configured as depicted in FIGS. 4a and4 b, it controls the optical paths of the light beam pair so that theyare directed to either output port 504 (FIG. 4a) or output port 503(FIG. 4b) depending on the control state of the second polarizationrotator array 101. When the first polarization rotator array 100 isswitched to its complementary control state, shown in FIGS. 4c and 4 d,the light beam pair is directed to either output port 502 (FIG. 4c) oroutput port 501 (FIG. 4d) depending on the control state of the secondpolarization rotator array 101. The final polarization rotator array102, changes the polarization of one of the beams by 90° so that thebeams return to orthogonal polarizations and are then recombined by thefinal birefringent element 60, as previously discussed.

With both of the designs disclosed above, two design considerationshould be kept in mind. First, the On and Off characteristics of eachsub-element in the polarization rotator arrays are controlled digitally(e.g., “1” for On and “0” for Off). Second, there are a total of 2^(N)output ports when N stages of birefringent elements and polarizationrotator arrays are placed in series. Each of the stages produces twopossible output directions. Based on these design concepts, a digitallyprogrammable optical routing switch can be realized. A control statetable is provided in Table 1.

Another key feature of the present design is its fault tolerance. Thiscan be better understood by considering FIGS. 3a-3 a and 4 a-4 d. Inboth sets of figures, the polarization rotator arrays are shown withfilled and unfilled squares to represent polarization rotation and nopolarization rotation, respectively, for each sub-element. For example,the last polarization controller 102 in both figures has four pairs ofsub-elements. The sub-elements in each pair to controlled incomplementary states (i.e., when one sub-element is on and the othersub-element in the pair is off). As shown in the figures, the four pairsof sub-elements are arranged such that only the pair intercepting thelight beams has its upper sub-element set for vertical polarization andits lower sub-element set for horizontal polarization. The other threepairs are set to complementary states so that the polarization of anyleaked optical energies are turned to the opposite polarization and arethereby sent in the wrong direction by the birefringent elements. Forexample in FIG. 3a, the eight pixels of the third polarization rotatorarray 102 are set to On, Off, Off, On, On, Off, On, Off from top todown. If we compare the pixels in FIG. 3b, 3 c and 3 a, except the firsttwo pixels, this combination is the reverse of these control states inwhich light beams can be coupled to those three ports. This blockingassures low cross-talk between the output channels.

Here again, this embodiment can also be viewed as a tree structure of1×2 optical switches receiving the beam pair exiting the firstbirefringent element 30. Both stages in the tree structure include apolarization rotator array 100, 101 that selectively rotates thepolarization of the beam pair so that both beams have the samepolarization determined by the control state, and a birefringent element40, 50 that selectively routes the beam pair along either of twoalternative optical paths determined by their polarization. The finalpolarization rotator array 102 rotates the polarization of the beam pairso that they are orthogonally polarized, and the final birefringentelement 60 combines the orthogonally-polarized beams at the desiredoutput port 501-504.

3-D DESIGN OF A 1×4 OPTICAL ROUTING SWITCH. FIGS. 5a through 5 d show athree-dimensional structure for a 1×4 optical switch. Here, the secondand third birefringent elements 40 and 50 have been oriented at 90° withrespect to the first and fourth birefringent elements. The opticalsignal from the input port 500 enters the first birefringent element 30and is split into horizontally and vertically polarized components. Inthe following figures, double-headed lines parallel to the base plane ofthe setup represent horizontal polarization, whereas double-headed linesperpendicular to the base plane represent vertical polarization.

FIG. 5a depicts an optical routing switch configured to route the inputsignal to output port 501. In FIG. 5a, the polarization rotator 100 hasthe state of (On, Off) that changes the horizontally-polarized beam tovertical polarization. The two light beams then carry the same verticalpolarization at the exit of the first polarization rotator array 100.These two vertical polarizations are considered to be extraordinarywaves when passing through the second birefringent element 40 andtherefore propagate upward. The second polarization rotator array 101intercepts the beam pair but applies no polarization rotation so thatthe both beams maintain vertical polarization. The beams then enter thethird birefringent element 50 and again propagate upward. The twovertical polarizations pass through a third polarization rotator array102, which rotates the polarization of one of beams by 90° so that theybecome orthogonal again. The orthogonally-polarized beam pair isrecombined by the fourth birefringent element 60 and exit at output port501.

In FIG. 5b, input port 500 is coupled to output port 502. Again, thesame vertical polarizations result after the first polarization rotatorarray 100, as in the case of FIG. 5a. The beams propagate upward andexit at the higher level of the second birefringent element 40. In thecase of FIG. 5b, the second polarization rotator array 101 is set to“On” and the polarizations of both beams are rotated by 90 degrees(i.e., both beams become horizontally polarized). The horizontally-polarized beams are considered to be ordinary waves in the thirdbirefringent element 50 and therefore propagate straight through thiselement 50. The final polarization rotator array 102 intercepts the twobeams and rotates the polarization of one of the beams by 90 degrees sothat the beam pair will be recombined by the fourth birefringent element60 and exit at output port 502.

In FIG. 5c, input port 500 is coupled to output port 503. Thesub-elements of the first polarization rotator array 100 are switched tothe complementary states from those of the previous two cases. Thisresults in horizontal polarizations when the beam pair passes throughthe first polarization rotator array 100. Both beams propagate straightthrough the second birefringent element 40 (at its lower level) becausethey are considered as ordinary waves in the birefringent element 40.The polarizations of both beams are rotated by 90° by the secondpolarization rotator array 101, so that they become verticallypolarized. These vertically-polarized beams are considered to beextra-ordinary waves in the third birefringent element 50 and propagateupward. The final polarization rotator array 102 intercepts the twobeams and rotates the polarization of one of the beams by 90° such thatthey become orthogonally polarized. The beams are recombined by thefourth birefringent element 60 and exit at output port 503.

In FIG. 5d, input port 500 is coupled to output port 504. In this case,the first polarization rotator array 100 is set to the same controlstate as in FIG. 5c. This results in horizontal polarizations when thebeam pair passes through the first polarization rotator array 100. Thebeams propagate straight through the second birefringent element 40 (atits lower level) because they are considered as ordinary waves. In thisfinal control state, the polarization controller 101 is set to apply nopolarization rotation to the light beams. The horizontally-polarizedbeams are considered to be ordinary waves in the third birefringentelement 50 and propagate straight through this element 50. The finalpolarization controller 102 intercepts the two beams and rotates thepolarization of one of the beams by 90 degrees. Theorthogonally-polarized beams are recombined through the fourthbirefringent element 60 and exit at output port 504.

FIGS. 6a through 6 d show another 3-D design of a 1×4 routing switch. Inthis case, the order of the second and third birefringent elements isreversed in comparison to those in FIGS. 5a through 5 d. Here, the firstpolarization rotator array 100 combines with the second birefringentelement 50 to determine whether the beams are directed to either a firstset of output ports 501, 503 or a second set of output ports 502, 504.The combination of the second polarization rotator array 101 and thethird birefringent element 40 determine whether the beams are directedto either output port 501 or 503 in the first set, or output port 502 or504 in the second set.

FIGS. 7a through 7 d are diagrams of yet another 3 a dimensionalembodiment of a 1×4 optical routing switch in accordance with thepresent invention. All the birefringent elements have the samethickness.

As can be understood from the above designs, these routing switches areworkable when the polarization rotators intercepting the light beam pairare set to the correct control states. All other sub-elements can beleft afloat or switched to arbitrary control states. However, tomaintain high-performance and low crosstalk for the routing switch, thesub-elements or pixels are carefully arranged such that the total effectto block optical leakage to the output ports is maximized. For example,as shown in FIGS. 6a-6 d, pixels of the third polarization rotator array102 are set to be complementary to their original state. Therefore, anyundesired optical leakage from incomplete polarization rotation will berotated into wrong polarizations and will not be coupled to the outputports, thereby minimizing crosstalk.

1×N ROUTING SWITCH USING POLARIZED BEAMSPLITTERS. In the previouslydiscussed embodiments, optical signal routing is obtained through use ofa tree architecture. In those cases, each of the optical switchingstages redirects the optical signal into either of two possible opticalpaths. As the signal propagates through the switch, N stages result in2^(N) possible output ports. In contrast, the following examples of 1×Nswitches (where N is an arbitrary number) shown in FIGS. 8 through 12illustrate switches using a series architecture. In these switches,polarization beamsplitters (PBS) are used in place of birefringentelements as polarization-dependent routing elements.

A polarized beamsplitter (PBS) permits light of a predeterminedpolarization to pass directly through the beamsplitter, butorthogonally-polarized light is reflected or refracted within thebeamsplitter and exits along a separate optical path. This is typically90 degrees from the first beam, as shown in FIGS. 8 and 10.

FIG. 8 illustrates the structure for a 1×5 optical switch. Thebirefringent elements used in the previous cases have been replaced bypolarized beamsplitters 801, 802, 803, and 804. Each PBS is coupled witha polarization rotator 700, 705, 706, and 707 that rotates thepolarization of the beam pair accordance with the control state of theoptical switch. Each pair of a polarization rotator and a PBS can beconsidered as a 1×2 optical switching stage. For example, thepolarization rotator 705 controls the state of polarization of the beampair to be either vertical or horizontal. The following PBS 802 eitherroutes the beam pair to output port 502 or passes it through to the nextstage for further routing. The polarization separation and recombinationat the input and output ports are the same as before, using abirefringent element 30, 601, 602, 603, 604, and 605 in combination witha double-pixel polarization rotator array 700, 701, 702, 703, 704, and708 for orthogonal polarization control.

Due to the typical low polarization extinction ratio of a PBS, a seriesof optional polarizers 901, 902, 903, 904, and 905 are used at each ofthe output ports 501 through 505 in the embodiment depicted in FIG. 8.These high extinction ratio polarizers (e.g., a Polarcor dichroic glasspolarizer with a polarization extinction ratio of 10000:1) purify thepolarization to reduce cross-talk. It is noted, however, the switch canperform its basic function without the use of these polarizers.

FIG. 10 provides an example of a 1×8 switch using two series ofswitching stages based on polarized beamsplitters. Each stage consistsof a polarization rotator 700, 711-713, and 715-717 in combination witha PBS 800-807 to create a 1×2 optical routing switch. The first PBS 800routes the beam pair to either output ports 501-504 or 505-508, based onthe control state of the two pixels in the first polarization rotatorarray 700. The operation of the remainder of the two series of stages inthis 1×8 switch is similar to that described above and shown in FIG. 8.

FIG. 11 illustrates a 1×4 optical routing switch using another type ofpolarization-dependent routing element 801, 802, and 803 that has anoffset angle at the output for vertical and horizontal polarizations.This type of polarization separator 801-803 can be regarded as acompromise between the properties of a birefringent element (i.e.,parallel beam output, high extinction ratio) and a PBS (perpendicularbeam outputs, low extinction ratio). It provides a high polarizationcontrast ratio and also separated the output beams at an angle.

This feature relaxes some of the practical constraints in fabricatingthe device, such as packaging of the Grinlens at the output port. If a1×8 switch is constructed using birefringent elements so that the twoorthogonal polarizations are parallel to each other, three birefringentelements are needed having thicknesses of d, 2 d, and 4 d, respectively.With the current Grinlens size of 1.8 mm (which defines the minimumdistance between the output ports), the minimum thickness (d) of thefirst birefringent element is 18 mm. With a total thickness of 7 d(d+2d+4d), this is equal to 126 mm. The total minimum optical pathlength is then on the order of about 130 mm with all other componentsinserted into the device. This long coupling distance will cause largeinsertion loss and is difficult to manufacture. Although this problemcan be resolved through the use of right angle prism that deflects thelight at the output, this approach further increases the cost andcomplexity of the device. The use of a PBS or angled beam separator canrelax this coupling restraint because the output angle further separatesthe optical paths so that the geometric increase in the size of theangled beam separator is no longer required. The result is a morecompact switch having a smaller loss.

FIG. 9 is an example of a 1×8 optical routing switch using a network of1×2 switching stages forming a binary tree structure. Here again, eachstage consists of a polarization rotator 100, 101, 102, 103, 104, 105,106 in combination with an angled polarization separator 801, 802, 803,804, 805, 806, and 807. The input beam is separated into a pair oforthogonally-polarized beams by a first birefringent element 30, as inthe previous embodiments. The polarization of one of theseorthogonally-polarized beams is rotated by 90 degrees by the firstpolarization rotator array 100 so that both beams have the samepolarization, as determined by the control state of the switch. The beampair is routed through the network of 1×2 switching states based on thecontrol states of the polarization rotators associated with each stage.It should be noted that the first polarization rotator array 100 has twopixels, while the remaining polarization rotators 102 through 106require only a single pixel. At each output port, a final polarizationrotator array 107-114 returns the beam pair to orthogonal polarizationsso that they can be combined by the final birefringent element 601-608.

FIG. 13 is a cross-sectional diagram of a polarization-dependent routingelement 31 that could be used in place of the birefringent elements 30and 60 to separate the input beam into orthogonally-polarizedcomponents, or to combine the orthogonally-polarized beams at the outputport. This polarization-dependent routing element 31 is a combination ofa PBS with a reflective prism. The vertically-polarized component of theinput beam passes directly through the element 31. However, thehorizontally-polarized component of the input beam is reflected by 90degrees within the PBS and reflected a second time by the reflectivesurface of the prism so that the horizontally-polarized beam emergesparallel to, but separated from the vertically-polarized beam.

N×M ROUTING SWITCH USING POLARIZED BEAMSPLITTERS. The embodiments of thepresent invention depicted in FIGS. 8 through 11 are 1×N routingswitches having a single input port and N output ports. This concept canbe extended to create optical routing switches having an arbitrarynumber of input and output ports.

FIG. 12 illustrates an implementation in which four 1×4 switches arestacked in parallel on top of one another. In one network applicationusing a N×N structure, a total of 2N of the 1×N modules are needed. Inthe 4×4 case, eight of the 1×4 switching modules are required. With thefour-level architecture shown in FIG. 12, two of these devices aresufficient to construct a 4×4 crossbar switch. From the material costperspective, because the optical components in the switch are the sameexcept for increased size in one dimension, material costs remainvirtually unchanged. This rapidly decreases the average material costper level as the number of levels increases.

FIG. 14 is a cross-sectional diagram of an alternative embodiment of a2×2 routing switch using a single PBS 800. Both input ports 500, 501include a birefringent element 600, 601 that spatially separates theinput beam into a pair of orthogonally-polarized beams. The polarizationrotator arrays 700 and 701 rotate the polarization of at least one ofthe beams so that both beams are either horizontally or verticallypolarized, determined by the control state of the switch. Depending ontheir polarization, the beams either pass directly through the PBS tothe opposing output port 502 or 503, or are reflected by 90° to theother output port 503 or 502. Both output ports 502, 503 include apolarization rotator array 702, 703 that returns the beam pair toorthogonal polarizations. A final birefringent element 602, 603 combinesthe orthogonally-polarized pair of beams at the output port 502, 503.

FIG. 15 is a cross-sectional diagram of an alternative embodiment of a2×2 routing switch that extends the basic concept of the embodimentdepicted in FIG. 14 to a two-dimensional network of polarizedbeamsplitters 800, 801, 802, and 803. Each of the polarizers 901, 902,903 and 904 can be externally controlled in accordance with the controlstate of the switch to adjust the polarization of the beam pair enteringthe polarized beamsplitters 801, 802, and 803. The polarization of thebeam pair entering the initial PBS 800 is controlled by the polarizationrotator arrays 700 and 701. As before, both output ports 502, 502include a polarization rotator array 702, 703 that returns the beam pairto orthogonal polarizations, and a final birefringent element 602, 603that combines the orthogonally-polarized pair of beams at the outputport 502, 503.

FIG. 16 is a cross-sectional diagram of a 2×8 routing switch usingmultiple polarized beamsplitters 800 through 807 in a seriesarchitecture similar to the 1×8 routing switch shown in FIG. 10. Asecond input port has been added to the initial PBS 800, which functionsin the same manner as the embodiments shown in FIG. 14 and 15.

FIG. 17 is a cross-sectional diagram of a 4×4 routing switch using atwo-dimensional network or array of 16 polarized beamsplitters 801, 802,803, etc. As before, each of the input ports 501 through 504 includes abirefringent element 601-604 that spatially separates the input beaminto a pair of orthogonally-polarized beams, and a polarization rotatorarray 701-704 that rotates the polarization of at least one of the beamsso that both beams are either horizontally or vertically polarized,determined by the control state of the switch. The beam pair then entersthe network of polarized beamsplitters, where they are routed to thedesired output port 505-508 by controlling the states of the appropriatepolarization rotator arrays 701-704 and polarizers 901, 902, 903, etc.,associated with each of the polarized beamsplitters 801, 802, 803, etc.Each output port includes a polarization rotator array 705-708 thatreturns the beam pair to orthogonal polarizations, and a finalbirefringent element 602, 603 that combines the orthogonally-polarizedpair of beams at the output port.

FIG. 18 is a cross-sectional diagram of an alternative embodiment of a4×4 routing switch. The locations of the four input ports 501 through504 and the output ports 505 through 508 have been shifted relative tothe 4×4 network of polarized beamsplitters 801, 802, 803, etc. However,the functionality of this embodiment is essentially the same as the 4×4routing switch shown in FIG. 17.

FIG. 19 is a cross-sectional diagram of a 6×6 routing switch using a 4×4network of polarized beamsplitters 801, 802, 803, etc. A 4×4 array ofpolarized beamsplitters could support up to a maximum of 16 input/outputports. However, isolation and overlap of the beam paths become problemsas the number of input/output ports increases. This embodiment uses all16 ports, but the beams exiting at two of the ports (see birefringentelements 621 and 622) are routed as inputs to two other ports (seebirefringent elements 623 and 624) and can then be directed to any ofthe output ports 507 through 512, as previously discussed. Thisconfiguration helps to reduce the number of PBSs and polarizers, andthereby reduces manufacturing costs.

FIGS. 20a and 20 b are diagrams of the two control states of analternative embodiment of a 2×2 routing switch that uses a 2×2 networkof PBSs 801, 802, 803, and 804. Both of the input ports include abirefringent element 601, 602 that spatially separates the input beaminto a pair of orthogonally-polarized beams, and a polarization rotatorarray 701, 702 that rotates the polarization of at least one of thebeams so that both beams are either horizontally or verticallypolarized, determined by the control state of the switch. The beam pairthen enters the network of PBSs 801-804, where they are routed to thedesired output port. Both of the output ports include a polarizationrotator array 703, 704 that returns the beam pair to orthogonalpolarizations, and a final birefringent element 603, 604 that combinesthe orthogonally-polarized pair of beams at the output port. FIG. 20aillustrates the first control state of the routing switch wherein thepolarization rotator arrays 701 and 702 associated with the both inputports change the polarization of the input beam pairs so that they passdirectly through PBSs 801 and 802. In contrast, FIG. 20b illustrate thesecond control state in which the polarization rotator arrays 701 and702 change the polarization of the beam pairs so that they are reflectedby 90° within PBSs 801 and 802. The beam pairs are also reflected by 90°within PBSs 804 and 803 to route the beam pair to the opposite set ofoutput ports from those shown in FIG. 20a.

FIG. 21 is a diagram of an alternative embodiment of a 4×4 routingswitch employing a network of PBSs 801-808 and polarizers 901-911 toroute the beam pairs from each input port to the desired output ports.As before, each of the input ports includes a birefringent element601-604 that spatially separates the input beam into a pair oforthogonally-polarized beams, and a polarization rotator array 701-704that rotates the polarization of at least one of the beams so that bothbeams are either horizontally or vertically polarized, determined by thecontrol state of the switch. The beam pair from each input port thenenters the network of PBSs 801-808 and polarizers 901-911, where theyare routed to the desired output port 508 by controlling the states ofthe appropriate polarization rotator arrays 701-704 and polarizers901-911. Each output port includes a polarization rotator array 705-708that returns the beam pair to orthogonal polarizations, and a finalbirefringent element 605-608 that combines the orthogonally-polarizedpair of beams at the output port.

N×M ROUTING SWITCH USING RIGHT-ANGLE PRISMS. FIGS. 22a and 22 b are twoside views and FIGS. 22c and 22 d are two top views of the two controlstates of an alternative embodiment of a 2×2 routing switch using a pairof right-angle prisms 223 and 224. In the first control stateillustrated in FIGS. 22a and 22 c, the input beam from input port 501 isrouted to output port 500, and the input beam from input port 502 isrouted to output port 503. In the second control state shown in FIGS.22b and 22 d, the input beam from input port 501 is routed to outputport 503, and the input beam from input port 502 is routed to outputport 500. Here again, birefringent elements 30, 60 are associated withthe input ports 501, 502 and output ports 503, 504. These birefringentelements 30, 60 spatially separate the input beam from either input portinto a pair of orthogonally-polarized beams. A polarization rotatorarray 221 or 226 rotates the polarization of at least one of the beamsso that both beams are either horizontally or vertically polarized,determined by the control state of the switch. The beam pair then entersa second birefringent element 222 or 225, which either allows the beampair to pass directly through the second birefringent element 222, 225in the same horizontal plane (as shown in FIGS. 22a and 22 c), ordirects the beam pair upward to a second vertical plane (as shown inFIGS. 22b and 22 d), based on the polarization of the beam pair.

In the first control state depicted in FIGS. 22a and 22 c, the beam pairis reflected twice within a right-angle prism 223 or 224, as shown inFIG. 22c, and thereby directed back toward the adjacent output port 500or 503. The polarization rotator array 220 or 227 associated with eachoutput port 500 or 503 returns the beam pair to orthogonalpolarizations, and birefringent element 30 or 60 combines theorthogonally-polarized pair of beams at the output port.

In the second control state shown in FIGS. 22b and 22 d, the secondbirefringent element 222 or 225 directs the beam pair upward to a secondvertical plane so that they pass above the prisms 223 and 224 directlyto the second birefringent element 222 or 225 on the opposite side ofthe device. The second birefringent element 222, 225 return the beampair to the lower vertical plane due their polarization. As before, thepolarization rotator array 220 or 227 associated with each output port500 or 503 returns the beam pair to orthogonal polarizations, andbirefringent element 30 or 60 combines the orthogonally-polarized pairof beams at the output port.

The present invention has the advantages of: (1) polarization-dependentoperation; (2) low inter-channel crosstalk; (3) low insertion loss; (4)operation over a broad ranges of wavelengths; (5) a wide range ofoperating temperatures; (6) switching speeds varying from millisecondsto nanoseconds when using different polarization converters; and (7) ascaleable structure (M×N) that allows multiple switches to be stackedtogether. These switch structures are best implemented withliquid-crystal polarization rotators, because of their spatial lightmodulation characteristics. In such a case, pixelized modulators can beused to control the beam pair at each stage. A large matrix can befabricated into one structure that results in a large-scale N×M opticalrouting switch.

The above disclosure sets forth a number of embodiments of the presentinvention. Other arrangements or embodiments, not precisely set forth,could be practiced under the teachings of the present invention and asset forth in the following claims.

We claim:
 1. A method of switching optical signals, comprising: separating an optical input signal into a pair of at least approximately orthogonally polarized beam components; selectively changing the polarization of one of the at least approximately orthogonally polarized beam components of the input signal such that the beam components comprise primarily a first polarization or a second polarization; receiving the beam components at a first polarization beamsplitter; directing the beam components for receipt by a second polarization beamsplitter if the beam components comprise primarily the first polarization; and selectively changing the polarization of the beam components if the beam components comprise primarily the first polarization to effect whether the second polarization beamsplitter directs the beam components for receipt by a first output port or a second output port.
 2. The method of claim 1, further comprising: receiving the beam components from the second polarization beamsplitter; changing the polarization of at least one of the beam components so that one component is at least approximately orthogonal to the other component of that signal; and combining the at least approximately orthogonally polarized components to form a single output beam.
 3. The method of claim 1, further comprising: directing the beam components for receipt by a third polarization beamsplitter if the beam components comprise primarily the second polarization; and selectively changing the polarization of the beam components if the beam components comprise primarily the second polarization to effect whether the third polarization beamsplitter directs the beam components for receipt by a third output port or a fourth output port.
 4. The method of claim 3, further comprising: receiving the input signal from the third polarization beamsplitter; changing the polarization of at least one of the beam components so that one component is at least approximately orthogonal to the other component of that signal; and combining the at least approximately orthogonally polarized components to form a single output beam.
 5. A method of switching optical signals, comprising: separating each of a plurality of optical input signals into a pair of at least approximately orthogonally polarized beam components; selectively changing the polarization of one of the at least approximately orthogonally polarized beam components of at least one input signal such that the beam components comprise primarily a first polarization or a second polarization; receiving the beam components of the at least one input signal at a first polarization beamsplitter of a network of polarization beamsplitters; communicating the beam components of the at least one input signal to another polarization beamsplitter of the network of polarization beamsplitters; and selectively altering the polarization of the beam components of the at least one input signal to affect the path of those beam components through the network of polarization beamsplitters and to determine which one of a plurality of output ports will receive the beam components.
 6. The method of claim 5, wherein selectively altering the polarization of the beam components of the at least one input signal to affect the path of those beam components through the network comprises passing the beam components of the at least one input signal through a liquid crystal polarization rotator residing between two adjacent polarization beamsplitters within the network.
 7. The method of claim 5, wherein selectively altering the polarization of the beam components of the at least one input signal to affect the path of those beam components through the network comprises passing the beam components through a liquid crystal polarization rotator before communicating the beam components to the another polarization beamsplitter of the network of polarization beamsplitters to affect the path of the beam components through the network. 